1. Field of the Invention
The present invention relates generally to radio frequency identification (RFID) tags.
2. Description of the Related Art
Many product-related and service-related industries entail the use and/or sale of large numbers of useful items. In such industries, it may be advantageous to have the ability to monitor the items that are located within a particular range. For example, within a particular store, it may be desirable to determine the presence of inventory items located on the shelf, and that are otherwise located in the store.
A device known as an RFID xe2x80x9ctagxe2x80x9d may be affixed to each item that is to be monitored. The presence of a tag, and therefore the presence of the item to which the tag is affixed, may be checked and monitored by devices known as xe2x80x9creaders.xe2x80x9d A reader may monitor the existence and location of the items having tags affixed thereto through one or more wired or wireless interrogations. Typically, each tag has a unique identification number that the reader uses to identify the particular tag and item.
Currently available tags and readers have many disadvantages. For instance, currently available tags are relatively expensive. Because large numbers of items may need to be monitored, many tags may be required to track the items. Hence, the cost of each individual tag needs to be minimized. Furthermore, currently available tags consume large amounts of power. Currently available tag power schemes, which include individually tag-included batteries, are inefficient and expensive. These inefficient power schemes also lead to reduced ranges over which readers may communicate with tags in a wireless fashion. Still further, currently available readers and tags use inefficient interrogation protocols. These inefficient protocols slow the rate at which a large number of tags may be interrogated.
Hence, what is needed is a tag that is inexpensive, small, and has reduced power requirements. Furthermore, what is needed are more efficient tag interrogation techniques, that operate across longer ranges, so that greater numbers of tags may be interrogated at faster rates.
The present invention is directed to a charge pump that is capable of converting a high frequency signal to a substantially direct current (DC) signal. The charge pump includes an input capable of receiving a high frequency signal, and a plurality of stages parallel coupled to the charge pump input. Each of the parallel stages has a first capacitor coupled between the charge pump input and a central node in the stage. A first diode has an anode coupled to the central node and a cathode coupled to a second node in the stage. A second diode has an anode coupled to the second node in a prior stage and a cathode coupled to the central node in the stage, except for in the first stage which has the anode coupled directly to ground. A second capacitor is coupled between the second node and ground. A third diode has an anode coupled to the central node and a cathode coupled to ground. A charge pump output is coupled to the second node in a last stage of the plurality of stages.
During operation, charge from the high frequency signal is accumulated in the plurality of stages during a first half cycle of the high frequency signal, and is passed from a nth stage of the plurality of stages to a (n+1)th stage of the plurality of stages during a second half cycle of the high frequency signal, the (n+1)th stage being closer to the charge pump output than the nth stage. The accumulated charge increases as it moves through the plurality of stages to the charge pump output to produce a DC output voltage that is sufficiently stable to be utilized as a power supply. In embodiments of the invention, the charge pump provides a power supply for an RF identification tag.
The first diode has a first threshold voltage VTH1, the second diode has a second threshold voltage VTH2, and the third diode has a third threshold voltage VTH3. The third threshold voltage is greater than at least one of the first threshold voltage and the second threshold voltage. In one embodiment, the third threshold voltage is greater than both the first threshold voltage and the second threshold voltage. The third diode operates to reduce the efficiency of the charge pump by conducting charge away from the central node when the voltage on the central node exceeds the threshold voltage of the third diode.
The first diode and the second diode can be configured using a metal oxide semiconductor field effect transistor (MOSFET) diode. The body and gate of the MOSFET diode can be coupled together to dynamically reduce the threshold voltage of the MOSFET diode, which enables the MOSFET diode to conduct in a low power environment. Furthermore, the gate of the second MOSFET diode can be forward-biased with the output of the first MOSFET diode. For example, the gate of the second MOSFET diode can be coupled to the second node, which is the output of the first MOSFET diode.
The third diode can include a plurality of stacked MOSFET devices, coupled between the central node and ground. The plurality of stacked MOSFET devices have a combined threshold voltage that is greater than that of a single MOSFET device in the plurality of stacked MOSFET devices.
In one embodiment, each stage includes a first capacitor coupled between the charge pump input and a central node. A first MOSFET diode is coupled between the central node and a second node. A gate and a drain of the first MOSFET diode is coupled to the central node, and a source of the second MOSFET diode is coupled to the second node. A second MOSFET diode is coupled between the second node of a prior adjacent stage and the central node. A drain of the second MOSFET diode is coupled to the second node of the prior adjacent stage, and a source of the second MOSFET diode coupled to the central node. A second capacitor is coupled between the second node and ground. A MOSFET device has a drain coupled to the central node and a source coupled to ground. A charge pump output is coupled to the second node in a last stage of the plurality of stages.
In one embodiment, the charge pump includes a first stage and a second stage. The first stage includes a first capacitor coupled between the charge pump input and a central node of the first stage. A first diode has an anode coupled to the central node and a cathode coupled to a second node in the first stage. A second diode has an anode coupled to ground and a cathode coupled to the central node. A second capacitor is coupled between the second node and ground. A third diode has an anode coupled to the central node and a cathode coupled to ground. A second stage of the charge pump includes a third capacitor coupled between the charge pump input and a central node of the second stage. A fourth diode has an anode coupled to the central node of the second stage and a cathode coupled to a second node of the second stage. A fifth diode has an anode coupled to the second node of the first stage and a cathode coupled to the central node of said second stage. A fourth capacitor is coupled between the second node of the second stage and ground. A sixth diode has an anode coupled to the central node of the second stage and a cathode coupled to ground. A charge pump output is coupled to the second node of the second stage.